Simplify the following expression: $n = \dfrac{r}{10} - \dfrac{r}{9}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10$ and $9$ $\lcm(10, 9) = 90$ $ n = \dfrac{9}{9} \cdot \dfrac{r}{10} - \dfrac{10}{10} \cdot \dfrac{r}{9} $ $n = \dfrac{9r}{90} - \dfrac{10r}{90}$ $n = \dfrac{9r -10r}{90}$ $n = \dfrac{-r}{90}$